QF use a 4 cabin aircraft but only sell it as a 3 class aircraft. Platinum FF get first choice of first class if on a business class ticket, rear lower deck business class if on a premium economy ticket, and rear premium economy class if on an economy ticket. Good value if you are travelling business class and can get a first class seat.

At this stage it is the 4 class aircraft with a three class cosmetic configuration. However, it will eventually go to the new three class config aircraft with A380 style seats and entertainment. Weather this will eventuate before the route swaps to Santiago is just a waiting game.

From the above link, its the second option with the config of 14 First; 66 Business; 40 Premium Economy; 187 Economy* that operate the EZE sectors.
If you can get the three class cosmetic config,
The premium economy seats sold as economy are rows 36 to 39 and if you can book it on the website and pay the $20 reservation fee it is well worth it. Just give it a try, the extra space makes such a big difference on a long flight. We got the premium seat on the way to EZE and a normal middle seat on the way home and the difference was amazing. We got to argentina feeling refreshed, while after the flight home, I was so sore and tired I needed two days off work to recover.